If you need that many rounds to put down an animal, you should be asking different questions.
Some of us may be looking to shoot more than one animal. I've had many, many occasions where I've shot several in quick succession. Culling pigs for example, I've often shot more than my magazine holds before the mob's got away, stuffing additional rounds in to get the stragglers.
I've also had occasion to cull buffalo with a double rifle, and often hunt with it or a single shot, combination or drilling. For all these purposes I think that the ammunition carrier on the butt of the rifle is invaluable. It is handy and fast to get additional rounds and, unlike some belt-mounted types, you aren't likely to damage the rifle on it.
I usually just use the elastic type - Allen I think. They seem to work well. I make a small hole for the rear swivel stud, which seems to hold them in place, and I've never had an issue with rounds falling out.
